A parametric method uses a type parameter to describe a relationship between its arguments. Instead of writing a separate search method for every element type, define one method that works across types while requiring the search value to match the collection’s element type. section 18.1
Listing 18.1 linearsearch in collection haystack for element needle
function linearsearch(haystack::AbstractVector{T}, needle::T) where T
for (i, x) in enumerate(haystack)
if needle == x
return i
end
end
nothing
end
In `linearsearch(haystack::AbstractVector{T}, needle::T) where T`, the `where` clause declares `T`. Its two appearances require the vector elements and the needle to share that type. section 18.1
A mismatch is rejected
`linearsearch([4, 5, 8], 5)` returns `2`. Searching the same vector for `"five"` produces a `MethodError`: this method does not accept a string needle with integer elements. section 18.1
A type parameter can also eliminate repetitive methods. `kindof(x::T) where T = T` returns the argument’s type without separate definitions for integers, strings, and every other type. For parametric types, think of `P{T}` as supplying a type to a template to obtain a concrete type. section 18.1 section 10.2.1
Listing 18.7 Point and Vec2D defined so the type parameter must be a number type
import Base: +, -
struct Point{T<:Number}
x::T
y::T
end
struct Vec2D{T<:Number}
Δx::T
Δy::T
end
"Adding vector `v` to point `p` creates a new point"
function +(p::Point{T}, v::Vec2D{T}) where T
Point(p.x + v.Δx, p.y + v.Δy)
end
"Subtracting two points gives a vector"
function -(p::Point{T}, q::Point{T}) where T
Vec2D(p.x - q.x, p.y - q.y)
end
The `Point` and `Vec2D` definitions constrain their type parameter to `Number`. Their addition method uses the same `T` for both arguments, so it requires matching coordinate types. section 18.3
Choose parameters to express the relationship you need. `Point{T}` and `Point{S}` allow two points to have different coordinate types; using `Point` without a parameter leaves that relationship unspecified. A broader method can instead constrain each parameter to an integer type with `Point{<:Integer}`. section 18.3
Generic methods also work alongside conversion and promotion. For mixed angle units, promoting both operands before arithmetic avoids defining an operation for every pair of units; `promote_rule` specifies the target type. This is a related reuse technique, not a requirement for every parametric method. section 9.3.7
